Semiconductor devices such as IGBTs can be used in many electrical systems as electronic switching elements for a variety of applications. For instance, IGBTs can be used in bridge circuits of a power converter to convert alternating current (AC) power to direct current (DC) power, and vice versa. Often a freewheeling diode is coupled in parallel with the IGBT to control current flow in, for instance, a bridge circuit.
IGBTs typically include three terminals, including a gate, a collector, and an emitter. The IGBT can be operated as a switching element by controlling the gate-emitter voltage using a gate drive circuit. For instance, when the gate-emitter voltage exceeds a threshold voltage for the IGBT, the IGBT can be turned on such that current can flow through the collector and emitter of the IGBT. When the gate-emitter voltage is less than the threshold voltage for the IGBT, the IGBT can be turned off such that current flow through the collector and emitter is limited.
During operation of the IGBT, it is important to turn the IGBT on and off quickly to reduce turn-off loss. Reducing the turn-off gate resistance associated with the IGBT can allow the IGBT to turn off quicker. During typical IGBT turn off, a parasitic miller capacitance from the gate-collector works in conjunction with the turn-off gate resistance to control the rate of voltage changes (dv/dt) of the collector-emitter voltage. However, a typical IGBT structure has inherent properties that limit the speed at which an IGBT can be turned off. More particularly, as explained in more detail below, when the gate-emitter voltage is negative with respect to a drift region of the IGBT, an adjoining drift region to a gate oxide layer tends toward inversion and becomes a shunt for displacement charge from the collector through the shunt to the emitter.
For instance, FIG. 1 depicts a plurality of example IGBT structures 100 that can be used in a variety of applications. The example IGBT structures 100 are provided for purposes of illustration and discussion. As shown, each IGBT structure 100 includes a gate 110, a collector 120, and an emitter 130. A gate oxide layer 150 is located adjacent to the gate 110.
Each IGBT structure 100 can include a drift region 135 where under blocking conditions, the majority of the voltage is accumulated. For increasing blocking on the IGBT, displacement current can flow to the gate 110 unless the gate 110 becomes oppositely biased with respect to the emitter 130, at which point that negative bias will force the like polarity carriers out of the drift region 135 away from the proximity of the gate oxide layer 150. Displacement currents then can use the channel 140 that is formed to the emitter 130, rather than using the gate drive as a way to travel to the emitter 130. The channel 140 that is formed provides a path or “shunt” connecting to a P+ region adjacent to the emitter 130. Having an inverted charge in the N− region near the gate oxide layer 150 can create a blocking region, causing the current from the miller capacitance to flow into the emitter 130 instead of the gate 150.
The presence of the inversion shunt or channel 140 can affect the miller capacitance of the IGBT during turn off. For instance, if the inversion shunt 140 is allowed to exist when the IGBT 100 is turning off and collector current is still flowing, the natural feedback of the miller capacitance of the IGBT 100 can be bypassed. This can reduce the effect of miller capacitance on the rate of voltage changes (dv/dt) of the collector-emitter voltage, allowing the IGBT 100 to potentially have an overvoltage of the collector-emitter voltage during turn off.
In addition, quicker turn on of the IGBT can lead to “snap off” behavior of a freewheeling diode coupled in parallel with the other IGBT of a phase leg during reverse recovery of the diode. The “snap off” behavior (i.e. high rate of change (di/dt) of reverse recovery current in the diode) can lead to voltage spikes. This in turn can lead to damage the diode and can ultimately cause a circuit failure.
Thus, a need exists for an gate drive circuit and method that can provide for improved control of the rate of voltage changes (dv/dt) of the collector-emitter voltage during semiconductor turn off. A gate drive circuit and method that provides for control of voltage change rates of the collector-emitter voltage during a period of time when a freewheeling diode is experiencing diode reverse recovery would be particularly useful.